Vote up?

I think a major issue that many commentators have already pointed out is that coding a mock-up from scratch (assuming you’re not using any libraries) is time consuming and makes assumptions about how the client will respond. Making visual comps or prototypes in an imaging or diagramming software speeds up the process of getting the layout and visual appearance approved without putting in a lot of time up front.

I’ve used Photoshop, Omnigraffle, and InDesign to do visual comps, as well as interactive prototypes. PS and Omnigraffle are great for small visual comps, but aren’t very efficient if you’re doing prototypes or comps/prototypes for large sites.

I really like the functionality that Fireworks promises. For those who are already familiar with other Adobe software, it takes very little time to become familiar with. However, it just doesn’t deliver. It’s extremely slow and buggy and much of the UI functionality you would expect isn’t currently offered. It’s a very immature application, but hopefully Adobe will get their act together with CS5. If they do, Fireworks is looking to be the best tool that provides designers a way to quickly produce slick interactive prototypes. Layers Magazine offers a great 10-step tutorial overview of Fireworks (Layers Magazine Adobe Fireworks Tutorial).